They accepted my Hong Kong phone number, which has a plus in it and has a different number of digits than a U.S. number. Either they're using better phone number validation than 95% of sites, or they're not validating the phone number format at all.

Edit: I was able to activate my account and log in with out getting a txt or anything, so presumably you could make up a fake phone number with the wrong number of digits, and no real person would get spammy calls.

You could always try setting your phone number to Bobby'); DROP TABLE STUDENTS --

Yeah it does not appear to be a *required field but still throws an error. I say its bad design. Just right click and inspect and delete that input field parent.. =) class='all_plan_group_row' to be specific